Arsenic sorption and redox reactions: Relevance to transport and remediation

Sorption and redox reactions strongly influence the toxicity, bioavailability, and potential mobility of arsenic (As) in soils. The objective of this paper is to describe the impact of soil flooding on the sorption and oxidation state of As. Soils were flooded in a batch system, and aqueous-phase measurements including Eh, pH, and dissolved Fe, Mn, and As were recorded at the end of each flooding period. In addition, aqueous- and solid-phase arsenite and arsenate were determined by ion chromatography and x-ray absorption spectroscopy. Soil suspension Eh decreased rapidly during the initial two days of flooding, thereby establishing anaerobic conditions for the remainder of the flooding period. Dissolved Fe and Mn levels increased, suggesting reductive dissolution of Fe and Mn (hydr)oxides occurred. Dissolved As decreased rapidly during the initial 24 h of the experiment, most likely the result of sorption. Oxidation state measurements showed dissolved arsenate disappeared with a concurrent increase in dissolved arsenite. Furthermore, solid-phase arsenate declined with a concurrent increase in arsenite. These aqueous and solid-phase observations indicate that arsenate is reduced to arsenite in flooded soils although it is not clear whether this reduction occurs in solution or on the soil particles.